This invention relates generally to electronic test instruments and in particular to a method for displaying only valid information by independently determining the validity of the signal being measured and responsively controlling the flow of measurement information to the instrument display.
Electronic measuring instruments are designed to aid in the troubleshooting and analysis of electronic circuits and systems. Such instruments include digital multimeters, oscilloscopes, and frequency timer/counters. The multimeter provides numerical information on voltage, current, resistance, temperature, and other parameters present at the probe tips. The oscilloscope provides a visual display of the signal waveform. The frequency timer/counter provides information on the frequency, periods, duty cycle, and other parameters present at the probe tips.
These instruments share the common design feature of having a pair of test probes for coupling the instrument to the nodes of a device under test. A design goal of the manufacturers of such instruments has been to provide the user with meaningful information regarding the parameter of interest. Meaningful information is information shown on the instrument display that accurately represents the condition of the valid, intended signal at the probes of the test instrument, free of extraneous disturbances. The display of information that is not meaningful is undesirable because it tends to mislead or confuse the user and requires an extra judgment as to whether the displayed information is indeed valid.
Recent trends in instrument designs spurred by changes in the electronics test market have resulted in portable test instruments that contain more functionality. The digital multimeter, oscilloscope, and frequency timer/counter having been combined into one portable, electronic test instrument which is optimized for service and repair applications. While the focus of the instrument designer is still on getting only meaningful information displayed, the working conditions of the instrument user have shifted. Many service and repair applications must be performed `on site`, rather than on a service bench in a repair facility.
The basic measurement process consists of the user acquiring a signal from the device under test and interpreting the meaningful information displayed by the test instrument. The device under test must be probed for signals often in physically cramped positions or in the presence of hazardous voltages, requiring the full attention of the user and often the use of both hands to manipulate the probes. The user must focus on first getting the proper contact with the instrument probes to the device being tested and then on interpreting the information displayed on the test instrument. To facilitate this two-step process of acquiring measurement data and then analyzing the information, the test instrument must be able to distinguish between invalid data, for example, when the probes are not in contact with a desired signal source, and valid measurement data, when meaningful information may be collected and displayed without physical intervention by the user. Valid measurement data may be obtained from a stable signal, which is defined as the continuous presence of the intended signal from the device under test during the relevant measurement period. The relevant measurement period is the predetermined time period in Which the test instrument is making measurements of the input signal. Measurements taken on a stable signal are thus valid and provide meaningful information. Conversely, an unstable signal is one that occurs during any lack of continuous contact by one or both probes resulting in the absence of the intended signal at any time during the relevant measurement period. Measurements taken on an unstable signal are invalid and provide information that is not meaningful and possibly confusing.
A method of displaying only meaningful measurement information must be able to distinguish three scenarios found during probing. First, the method must be able to distinguish invalid data when the probes are not in contact with an intended signal. Second, the method must distinguish invalid data when the probes first come into contact with the signal source because the signal is present only during a fraction of a regular measurement cycle in the test instrument, thereby producing an incomplete and possibly misleading measurement. Finally, the method must distinguish when the probes are removed from contact with the signal source to reject a measurement that is invalid because the signal is present only during a fraction of the regular measurement cycle thereby corrupting that measurement. By making the decisions in a correct and timely manner, meaningful information is thus maintained on the instrument display for the user.
A digital multimeter displays a selected measurement parameter, such as the voltage present between its probes, one measurement at a time. A primary tool of the service and repair technicians for many years, digital multimeters were the first instruments where displaying meaningful data for the user who must focus on probing as described above became an issue. The Touch-Hold.TM. feature of the Fluke 80-series digital multimeters is a stability determining method designed to hold a stable reading on the display until a new stable reading is encountered. The Touch-Hold.TM. function is the commercial embodiment of the method and apparatus disclosed in U.S. Pat. No. 4,532,470, READING SENDING METER, Jul. 30, 1985, to Thomas W. Wiesmann and assigned to Fluke Corporation. In accordance with the '470 patent, measurement data reasonably close to a predetermined zero level indicate the probes are not connected, the data are rejected as presumptively invalid, and the display is not updated. When the instrument probes contact a signal, an algorithm determines whether the measurement data are stable by comparing the incoming measurement data with upper and lower tolerance limits centered around a last stable measurement value. If the measurement data are within the tolerance limits, they are stable and the display is not updated. If the measurement data are outside the tolerance limits, the display is updated with the new measurement data, and the upper and lower tolerance limits are re-centered around the new measurement data as the last stable measurement value. The user is then free to remove the probes from the device under test and interpret the displayed information because the digital multimeter continues to display the last stable measurement data.
This method suffers from the disadvantage that it is not able to adapt to the speed of variation of the input signal to better discriminate unstable and stable signals. This disadvantage is most severe for slowly varying signals that may otherwise be considered stable. In this case, the input signal level slowly drifts outside the tolerance band, causing the display to update and the tolerance limits to be re-centered. Therefore, it would be desirable to provide a method of determining the stability of the signal that adapts to the speed of variation of the input signal so that slowly varying signals may be tracked and still be considered stable while allowing more rapid adjustment to input signals with a larger variation.
The digital storage oscilloscope (DSO) operates in a more complex fashion than the digital multimeter. Instead of one reading per measurement, the (DSO) collects a series of readings, referred to as a "waveform scan", at a predetermined rate in response to a trigger signal. In a process known as digital sampling, a waveform memory is filled with sample data from the input signal which are collected at a rate typically much higher than the measurement rate of a digital multimeter but at lower measurement resolution. The contents of the waveform memory can then be plotted to the instrument display, which may be liquid crystal display (LCD) or cathode-ray tube (CRT) for example, to display the waveform information to the user.
While all DSO's have a trigger control that can be set to selectively trigger only in the presence of a specified signal voltage level, such a trigger control typically does not have the necessary capability to distinguish valid from invalid data under the three probing scenarios discussed above to prevent the display of meaningless information. The problem of displaying only a meaningful waveform with a DSO is that a waveform is by nature dynamic over a short time period. For example, one cycle of a sine wave has an upper and a lower peak, making detection of invalid data collected during a scan over a relatively short time interval very difficult. A signal that is valid at the beginning of a scan may become invalid before the scan is finished, resulting in information being displayed that is not meaningful to the user.
A portable, electronic test instrument incorporating the measurement functions of a DSO, a frequency counter, and a DMM has been designed. The measurement functions may be conducted independently and simultaneously to produce separate signal parameter measurements of the same signal. Therefore, it would be desirable to provide a test instrument capable of measuring a parameter of a signal while independently measuring a second parameter of the signal to make a determination of its stability according to a stability determining method. In this way, only meaningful waveform information from the measurement would be displayed notwithstanding the intermittent coupling and decoupling of the test instrument to the signal during probing operations by the user.